From the prior art, vehicles with a parallel hybrid drive-train are known, in which addition of the torques of the internal combustion engine and at least one electric machine, that can be connected with the internal combustion engine, takes place preferably by means of a summation transmission, for example by a planetary transmission. In such a case the at least one electric machine can be connected to the belt drive or to the crankshaft of the internal combustion engine by means of a first clutch. The torques produced by the internal combustion engine and/or the at least one electric machine are transmitted, via a downstream transmission, to the driven axle. Further, it is known from the prior art to arrange the internal combustion engine and the electric machine in series.
According to the prior art, in parallel hybrid drive-trains an optional further clutch can be provided, which connects the transmission releasably to the drive unit consisting of the internal combustion engine and the electric machine, and which can serve as the starting clutch. In such a case the transmission can be connected releasably by the starting clutch to the electric machine.
In a parallel hybrid drive-train, depending on the operating strategy the drive mode of the vehicle can change between internal combustion engine-powered, electric machine-powered and mixed driving operation, and the situation can often arise that the internal combustion engine is started from a purely electric machine-powered drive mode.
From the prior art it is known to start up the internal combustion engine by means of a so-termed drag start; in this case, with the clutch located between the electric machine and the internal combustion engine engaged, the torque of the electric machine overcomes the drag resistance of the internal combustion engine so that the internal combustion engine is accelerated to its starting speed. Preferably, to carry out a drag start of the internal combustion engine the starting clutch provided between the electric machine and the vehicle's transmission is operated with a specified amount of slip.
When the starting clutch is in the engaged condition and slip is to be produced by reducing the torque to be transmitted by the starting clutch during its decoupling, i.e. by relaxing the rotationally fixed connection between the electric machine and the vehicle's transmission, for example because the internal combustion engine has to be started up by a drag start, then according to the prior art the starting clutch is controlled so as to reduce its current torque transmission far enough for the desired slip to be produced. To do this the starting clutch must be controlled in such a manner that the torque transmitted is smaller than the torque to be transmitted that corresponds to the slip desired.
Disadvantageously, this results in a delay in the drive-train because the clutch has to be changed from static friction to sliding friction. This delay can be perceived as unpleasant, particularly when the desired slip is only produced after a lengthy time interval, as can be the case for example when the torque to be transmitted is around 0 Nm; in such a case the torque transmitted by the starting clutch has to be greatly reduced.
From DE 10 2004 001 381 A1 by the present applicant a method is known for increasing the spontaneity of overlapping shifts in an automatic transmission, in which, with the shift command or immediately after it an engine ignition is specified, by which snatch opening of the shifting element provided in the transmission and to be disengaged and/or an increase of the speed gradient of the turbine rotation speed is achieved, the engine ignition taking place by the specification of a nominal speed to be produced or by the specification of a nominal engine torque to be produced.
Further, WO 2007/093249 A1 describes a method for decoupling at least one torque source from at least one other torque source in a drive-train which, to produce a drive output torque at its rear end, extends over a plurality of torque sources arranged one after another and connected mechanically to one another by rotating shafts, such that between at least one pair of torque sources is arranged a separator clutch which is disengaged to decouple the torque source positioned before the separator clutch from the remainder of the drive-train behind the separator clutch, and such that on either side of the separator clutch in each case at least one torque source can be controlled in relation to its torque provision to the drive-train to produce a required variation of the output torque.
In this known method it is provided that the at least two controllable torque sources arranged on different sides of the separator clutch are controlled during a decoupling preparation period before the separator clutch is disengaged, in such manner that the sum of the torques acting overall on the separator clutch at the end of the decoupling preparation period is raised and the required variation of the output torque is maintained during the decoupling preparation period. Preferably, the separator clutch releasably connects an internal combustion engine and an electric machine with one another.
By virtue of the method known from WO 2007/093249 A1 the jerky fluctuations of the output torque that occur when the internal combustion engine is decoupled, which can lead to likewise jerky (positive or negative) acceleration of the vehicle as a whole, should be reduced.
From EP 1 502 791 A2 a hybrid drive-train for a motor vehicle is known, in which a clutch is arranged between an internal combustion engine and a transmission, by means of which the internal combustion engine can be decoupled from the drive-train. In this case it is provided that when the clutch is to be disengaged, the torque transmitted by the clutch and optionally the gradient of the torque transmitted by the clutch are reduced before the clutch is disengaged.